Fly Ash and GGBS in Concrete
Supplementary cementitious materials (SCMs) are used across Indian concrete construction to partly replace cement. This page explains what fly ash and GGBS are, typical replacement levels, and what to check before using them.
What SCMs are
Supplementary cementitious materials partly replace cement in the mix and are counted within the total cementitious content, not treated as a separate ingredient. The two most common SCMs used in India are fly ash, a by-product of coal power generation used per IS 3812, and GGBS (ground granulated blast-furnace slag), a by-product of iron-making.
Typical replacement levels
Replacement levels are guidance and depend on the material and the project — they should be confirmed by trial. As a typical range, fly ash is commonly used at about 15–35% of the cementitious content, and GGBS at higher levels, commonly about 25–70%.
Benefits and the early-strength trade-off
Using fly ash or GGBS typically brings lower heat of hydration, improved long-term durability, and often lower cost and embodied carbon. The main caution is slower early-age strength gain, so early-strength requirements and curing need to be checked, and the exact replacement level confirmed by trial before use.
How this fits the calculator
In the calculator, an SCM is entered as a mineral admixture — a replacement percentage of the total cementitious content — so the water-cement ratio and cementitious content calculations both account for it.
Blended cement or site blending?
Fly ash and GGBS reach Indian sites in two ways. Factory-blended cements — PPC to IS 1489 (Part 1) with fly ash, and PSC to IS 455 with slag — are the most common. Alternatively, a ready-mix plant can add fly ash (IS 3812 Part 1) or GGBS (IS 16714) separately to OPC. Site blending gives more control over the proportion but needs weigh-batching and quality checks on the SCM; blended cement is simpler but fixes the proportion within the range allowed by its standard.
Curing and formwork striking
SCM mixes gain strength more slowly at early ages, especially in cool weather. IS 456 gives minimum formwork striking periods for OPC concrete and notes that they may need to be adjusted for other cements. With PPC, PSC or high-SCM mixes, plan for longer moist curing (IS 456 asks for at least 10 days where mineral admixtures or blended cements are used, against 7 days for OPC, and recommends extending this to 14 days) and check striking times against cube results rather than the calendar.
Worked example: counting fly ash in the cement content
A severe-exposure RCC mix needs at least 320 kg/m³ cementitious content and a w/c no higher than 0.45. A mix of 280 kg OPC + 70 kg fly ash (20% of 350 kg) with 157 litres of water gives a total cementitious content of 350 kg/m³ and a w/c of 157/350 = 0.45. Because IS 456 counts permitted mineral admixtures within the cement content, this mix meets both limits, provided the trial mixes achieve the target mean strength.
Frequently asked questions
Do SCMs replace cement completely?
No. They replace only part of the cement and are counted within the total cementitious content, not used on their own.
Do they reduce strength?
Early-age strength is usually slower to develop. Long-term strength and durability are often as good as, or better than, plain OPC concrete, but this should be confirmed by trial for your materials and mix.